The changes provide opportunities and risks for both shows: Kimmel can demand higher ad rates and potentially a larger audience, but will have to compete directly with top-rated "The Tonight Show With Jay Leno" on NBC and CBS's "The Late Show With David Letterman."

"Nightline," meanwhile, will lose its local news lead-in, but will gain primetime exposure, albeit on a little-watched night.

ABC News President sent a memo to staff Tuesday to assuage any potential concerns about "Nightline" moving to the lower-visibility timeslot. He said ABC "believes it has a stronger growth opportunity in late night if 'Jimmy Kimmel Live' precedes 'Nightline'" and that "growth is a primary objective of ABC and our news division."

"For now, it’s important to note that 'Nightline' has proven its ability to grow over three decades on the air with the show currently enjoying some of its highest ratings and best editorial work ever," he added. "The 'Nightline' team will now bring its excellent journalism to new time periods, and we especially welcome the chance to produce an hour every Friday in primetime, where new audiences will be introduced to the program’s signature storytelling, interviewing and investigations."

The shift marks another change in a late-night landscape that last dramatically shifted in 2010, when Jay Leno reclaimed "The Tonight Show" from Conan O'Brien, who later moved to TBS. "The Tonight Show" has since slipped in the ratings despite remaining the most watched show in total viewers and in the 25-54 demo most important to late-night advertisers.

About 20 "Tonight Show" staffers lost their jobs last week in layoffs Leno blamed on NBC's corporate parent, Comcast, during Monday night's monologue.